Just in case anyone WAS actually interested in this:
Local dealer has had a rummage around the parts database and now has the part on order for me, should be getting paint matched and fitted shortly after it arrives (it's on backorder, might take a week or two).
The part number is actually 4F0071640 9AX after all and it's listed in the database as fitting an S6, I assume that this means it'll fit pretty much any A6 saloon.
...Also on order is an S6 specific black grille to replace the platinum grey+chrome one already on the car. If anyone out there's interested in buying the one that comes of the car then PM me and we'll sort something out (it's in perfect condition).
...oh, took the plunge and got 20% tint on the glass (70% in the front for legal reasons). Will post some pics when all the other bits are fitted. Hopefully it'll look more like the V10 beast it should have done when it rolled out of the factory. The glass has already made a big difference.
next thing on the list is to persuade the wife to let me get some 20" Caractere rims. Somehow I don't think this will happen for a looooong time, if ever.